Carbon fiber composite material and method of producing the same

ABSTRACT

A method of producing a carbon fiber composite material includes: a first mixing step of mixing an elastomer and carbon nanofibers at a first temperature; and a second mixing step of mixing a mixture obtained by the first mixing step at a second temperature, and the first temperature is 50 to 100° C. lower than the second temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Division of application Ser. No. 11/133,204 filed May 20,2005, which in turn claims priority to Japanese Patent Application No.2004-151860 filed May 21, 2004. The disclosures of the priorapplications are hereby incorporated by reference herein in theirentirety.

BACKGROUND OF THE INVENTION

The present invention relates to a carbon fiber composite material and amethod of producing the same.

In recent years, a composite material using carbon nanofibers asdisclosed in Japanese Patent Laid-Open No. 10-88256 has attractedattention. Such a composite material is expected to exhibit improvedmechanical strength and the like due to inclusion of the carbonnanofibers. However, since the carbon nanofibers have strong aggregatingproperties, it is very difficult to uniformly disperse the carbonnanofibers in a matrix of a composite material. Therefore, it isdifficult to obtain a carbon nanofiber composite material having desiredproperties. Moreover, expensive carbon nanofibers cannot be efficientlyutilized.

SUMMARY

A first aspect of the invention relates to a method of producing acarbon fiber composite material, the method comprising:

a first mixing step of mixing an elastomer and carbon nanofibers at afirst temperature; and

a second mixing step of mixing a mixture obtained by the first mixingstep at a second temperature,

wherein the first temperature is 50 to 100° C. lower than the secondtemperature.

A second aspect of the invention relates to a carbon fiber compositematerial obtained by the above method.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 schematically shows a mixing method for an elastomer and carbonnanofibers utilizing an internal mixing method used in one embodiment ofthe invention.

FIG. 2 is a schematic view of an optical microscope image of anelastomer after a first mixing step in one embodiment of the invention.

FIG. 3 is a schematic view of an electron microscope (SEM) image of thecross section of a carbon fiber composite material after a second mixingstep in one embodiment of the invention.

FIG. 4 shows an SEM image of a composite material obtained in Example 2.

FIG. 5 shows an SEM image of a composite material obtained inComparative Example 3.

DETAILED DESCRIPTION OF THE EMBODIMENT

The invention may provide a carbon fiber composite material in whichcarbon nanofibers are uniformly dispersed, and a method of producing thesame.

An embodiment of the invention provides a method of producing a carbonfiber composite material, the method including:

a first mixing step of mixing an elastomer and carbon nanofibers at afirst temperature; and

a second mixing step of mixing a mixture obtained by the first mixingstep at a second temperature,

wherein the first temperature is 50 to 100° C. lower than the secondtemperature.

In this method of producing a carbon fiber composite material, theelastomer may have a molecular weight of 5,000 to 5,000,000. Theelastomer may be a nonpolar elastomer. The nonpolar elastomer may beEPDM.

According to this method of producing a carbon fiber composite material,the carbon nanofibers can be uniformly dispersed in the elastomer by thetwo stages of mixing steps. The first mixing step causes the carbonnanofibers to be dispersed over the entire elastomer by applying astrong shear force by mixing the elastomer and the carbon nanofibers ata temperature lower than the temperature of the second mixing step. Thecarbon nanofibers dispersed by the first mixing step may be scatteredover the elastomer as aggregates. In particular, when the elastomer is anonpolar elastomer such as EPDM, the carbon nanofibers tend to bedispersed in the elastomer in a state in which a number of small carbonnanofiber aggregates exist. The second mixing step causes the elastomermolecules to be cut to produce radicals by mixing the elastomer and thecarbon nanofibers at a temperature which is 50 to 100° C. higher thanthe temperature of the first mixing step. The aggregating force of thecarbon nanofibers can be reduced by causing the radicals of theelastomer molecules and the carbon nanofibers to bond, wherebynano-level dispersibility can be increased. As a result, the carbonfiber composite material in one embodiment of the invention has aconfiguration in which the carbon nanofibers are uniformly dispersed inthe elastomer as a matrix.

In this method of producing a carbon fiber composite material, the firsttemperature may be 0 to 50° C., and the second temperature may be 50 to150° C.

With this method of producing a carbon fiber composite material, thecarbon nanofibers may have an average diameter of 0.5 to 500 nm.

With this method of producing a carbon fiber composite material, thefirst mixing step may include mixing the elastomer and the carbonnanofibers together with carbon black.

The elastomer can be easily reinforced by the carbon black, which isless expensive than the carbon nanofibers, by mixing the elastomer andthe carbon nanofibers together with the carbon black in the first mixingstep. Moreover, the carbon nanofibers can be more uniformly dispersed bycomplicated flows of the elastomer occurring around the carbon blackduring the first and second mixing steps.

With this method of producing a carbon fiber composite material, thefirst mixing step may be performed by using an internal mixing method.

With this method of producing a carbon fiber composite material, thesecond mixing step may be performed by using an internal mixing method.

The use of the internal mixing method in the first and/or second mixingstep enables mass production, and the temperature management in themixing step can be accurately performed.

This method of producing a carbon fiber composite material may furtherinclude a third mixing step of mixing the carbon fiber compositematerial obtained by the second mixing step at a third temperature lowerthan the second temperature.

With this method of producing a carbon fiber composite material, thethird mixing step may include performing tight milling a plurality oftimes by using an open roll with a rotor interval of 0.5 mm or less.

Embodiments of the invention are described below in detail withreference to the drawings.

(A) Elastomer

The elastomer has a molecular weight of preferably 5,000 to 5,000,000,and still more preferably 20,000 to 3,000,000. If the molecular weightof the elastomer is within this range, since the elastomer molecules areentangled and linked, the elastomer easily enters the space between theaggregated carbon nanofibers to exhibit an improved effect of separatingthe carbon nanofibers. If the molecular weight of the elastomer is lessthan 5,000, since the elastomer molecules cannot be sufficientlyentangled, the effect of dispersing the carbon nanofibers is reducedeven if a shear force is applied in the subsequent step. If themolecular weight of the elastomer is greater than 5,000,000, since theelastomer becomes too hard, processing becomes difficult.

The carbon nanofiber generally has a structure in which the side surfaceis formed of a six-membered ring of carbon atoms and the end is closedby introduction of a five-membered ring. Since the carbon nanofiber hasa forced structure, a defect tends to occur, whereby a radical or afunctional group tends to be formed at the defect. Therefore, theelastomer and the carbon nanofiber can be bonded by using an elastomerhaving a high affinity (reactivity or polarity) to the radical of thecarbon nanofiber. For example, the inventors of the invention haveconfirmed that the carbon nanofibers can be more uniformly dispersed innitrile rubber (NBR) having a high polarity or in natural rubber (NR)containing a number of polar groups such as proteins. However, theinventors of the invention have also confirmed, by observation using anelectron microscope, that, although the carbon nanofibers can bedispersed over the entire elastomer when using a nonpolar elastomer suchas EPDM, many carbon nanofiber aggregates are scattered over theelastomer.

The nonpolar elastomers may be classified by the solubility parameter(SP value). As examples of the nonpolar elastomer, ethylene propylenerubber (EPDM, SP value: 16.0 to 17.8), styrene-butadiene rubber (SBR, SPvalue: 15.0 to 17.8), butyl rubber (IIR, SP value: 15.8 to 16.7),butadiene rubber (BR, SP value: 14.7 to 18.5), an olefin-basedthermoplastic elastomer (TPO, SP value: 17.5), and the like can begiven.

(B) Carbon Nanofiber

The carbon nanofibers preferably have an average diameter of 0.5 to 500nm, and still more preferably 0.5 to 100 nm. The carbon nanofiberspreferably have an average length of 0.01 to 1000 μm, and still morepreferably 50 μm or less.

The amount of the carbon nanofibers added is not particularly limited,and may be set depending on the application. The carbon fiber compositematerial in this embodiment may be directly used as an elastomermaterial in the form of a crosslinked elastomer, an uncrosslinkedelastomer, or a thermoplastic polymer, or may be used as a raw materialfor a metal or resin composite material. In the case of using the carbonfiber composite material in this embodiment as a raw material for ametal or resin composite material, the carbon fiber composite materialmay contain the carbon nanofibers in an amount of 0.01 to 50 wt %. Sucha raw material for a metal or resin composite material may be used as amasterbatch as a carbon nanofiber source when mixing the carbonnanofibers into a metal or a resin.

As examples of the carbon nanofibers, a carbon nanotube and the like canbe given. The carbon nanotube has a single-layer structure in which agraphene sheet of a hexagonal carbon layer is closed in the shape of acylinder, or a multi-layer structure in which the cylindrical structuresare nested. Specifically, the carbon nanotube may be formed only of asingle-layer structure or a multi-layer structure, or a single-layerstructure and a multi-layer structure may be present in combination. Acarbon material having a partial carbon nanotube structure may also beused. The carbon nanotube may be called a graphite fibril nanotube.

A single-layer carbon nanotube or a multi-layer carbon nanotube isproduced to a desired size using an arc discharge method, a laserablation method, a vapor-phase growth method, or the like.

In the arc discharge method, an arc is discharged between electrodematerials made of carbon rods in an argon or hydrogen atmosphere at apressure lower than atmospheric pressure to some extent to obtain amulti-layer carbon nanotube deposited on the cathode. When mixing acatalyst such as nickel/cobalt into the carbon rod and discharging anarc, a single-layer carbon nanotube is obtained from soot adhering tothe inner side surface of a processing vessel.

In the laser ablation method, a target carbon surface into which acatalyst such as nickel/cobalt is mixed is irradiated with strong pulselaser light from a YAG laser in a noble gas (e.g. argon) to melt andvaporize the carbon surface to obtain a single-layer carbon nanotube.

In the vapor-phase growth method, a carbon nanotube is synthesized bythermally decomposing hydrocarbons such as benzene or toluene in a vaporphase. As specific examples, a floating catalyst method, azeolite-supported catalyst method, and the like can be given.

The carbon nanofibers may be provided with improved adhesion to andwettability with the elastomer by subjecting the carbon nanofibers to asurface treatment such as an ion-injection treatment, sputter-etchingtreatment, or plasma treatment before mixing the carbon nanofibers andthe elastomer.

(C) Step of Dispersing Carbon Nanofibers in Elastomer

The step of dispersing the carbon nanofibers in the elastomer includes afirst mixing step of mixing the elastomer and the carbon nanofibers at afirst temperature, and a second mixing step of mixing the mixtureobtained by the first mixing step at a second temperature.

In this embodiment, an example using an internal mixing method isdescribed below as the first mixing step and the second mixing step.

FIG. 1 is a schematic view showing an internal mixer using two rotors.FIG. 2 is a schematic view of an optical microscope image of theelastomer after the first mixing step, in which the carbon nanofibersindicated by the black circle are partially magnified. FIG. 3 is aschematic view of an electron microscope image of the cross section ofthe carbon fiber composite material after the second mixing step.

In FIG. 1, an internal mixer 100 includes a first rotor 10 and a secondrotor 20. The first rotor 10 and the second rotor 20 are disposed at apredetermined interval, and cause the elastomer to be mixed by means ofrotation. In the example shown in FIG. 1, the first rotor 10 and thesecond rotor 20 are rotated in opposite directions (e.g. directionsindicated by the arrows in FIG. 1) at a predetermined velocity ratio. Adesired shear force can be obtained by adjusting the velocity of thefirst rotor 10 and the second rotor 20, the interval between the rotors10 and 20 and the inner wall of a chamber 70, and the like. The shearforce applied in this step is arbitrarily set depending on the type ofthe elastomer, the amount of the carbon nanofibers, and the like.

Pre-Mixing Step

An elastomer 30 is supplied to the internal mixer 100 through a materialsupply port 60, and the first and second rotors 10 and 20 are rotated.After the addition of carbon nanofibers 40 to the chamber 70, the firstand second rotors 10 and 20 are further rotated to mix the elastomer 30and the carbon nanofibers 40. A known compounding ingredient such asstearic acid may be added either simultaneously with or prior to theaddition of the carbon nanofibers 40. This step is generally calledbreakdown, in which the temperature of the internal mixer is set at 20°C., for example.

In this pre-mixing step, another compounding ingredient such as carbonblack 50 for reinforcement may be added either simultaneously with orprior to the addition of the carbon nanofibers 40 in an amount of 10 to100 parts by weight (phr), for example. A complex flow of the elastomer30 occurs around the carbon black 50 during mixing by adding the carbonblack 50, whereby the carbon nanofibers 40 can be more uniformlydispersed. As the carbon black 50, it is preferable to use carbon blackhaving a comparatively large average particle diameter of 40 to 500 nm.If the average particle diameter of the carbon black 50 is less than 40nm, since the processability becomes poor, a decrease in durability(deterioration) may occur due to internal friction. If the averageparticle diameter of the carbon black 50 is greater than 500 nm, theeffect of dispersing the carbon nanofibers may not be obtained duringmixing.

First Mixing Step

The first mixing step of further mixing the mixture obtained by mixingthe carbon nanofibers 40 and the carbon black 50 into the elastomer 30is performed. The first and second rotors 10 and 20 are rotated at apredetermined velocity ratio. In the first mixing step, the elastomerand the carbon nanofibers are mixed at the first temperature which is 50to 100° C. lower than the temperature in the second mixing step in orderto obtain a shear force as high as possible. The first temperature ispreferably 0 to 50° C., and still more preferably 5 to 30° C. If thefirst temperature is lower than 0° C., mixing becomes difficult. If thefirst temperature is higher than 50° C., since a high shear force cannotbe obtained, the carbon nanofibers cannot be dispersed over the entireelastomer. The first temperature may be set by adjusting the temperatureof the chamber 70 or the temperatures of the rotors 10 and 20. Thevelocity ratio and the temperatures may be controlled while measuringthe temperature of the mixture. In the case of performing the firstmixing step after the above-described mixing step using the sameinternal mixer, the internal mixer may be set at the first temperaturein advance.

In the case of using nonpolar EPDM as the elastomer 30, the carbonnanofibers 40 are dispersed over the entire elastomer 30 by the firstmixing step while forming aggregates (black circles shown in FIG. 2), asshown in the schematic view of the mixture in FIG. 2. The aggregate isformed of complexly entangled carbon nanofibers 40 as shown in thepartial enlarged view in FIG. 2. In FIGS. 2 and 3, the carbon black 50is not shown in order to clearly illustrate the dispersion state of thecarbon nanofibers 40.

Second Mixing Step

The mixture obtained by the first mixing step is supplied to anotherinternal mixer 100 to perform the second mixing step. In the secondmixing step, the mixture is mixed at the second temperature, which is 50to 100° C. higher than the first temperature, in order to produceradicals by cutting the molecules of the elastomer 30. The temperatureof the internal mixer used in the second mixing step is increased to thesecond temperature using a heater provided in the rotor or a heaterprovided in the chamber so that the second mixing step can be performedat the second temperature higher than the first temperature. The secondtemperature may be arbitrarily selected depending on the type of theelastomer used. The second temperature is preferably 50 to 150° C. Ifthe second temperature is lower than 50° C., since radicals are producedin the elastomer molecules to only a small extent, the carbon nanofiberaggregates cannot be disentangled. If the second temperature is higherthan 150° C., since the molecular weight of the elastomer isconsiderably decreased, the modulus of elasticity is decreased.

The period of time of the second mixing step may be arbitrarily setdepending on the second temperature, the rotor interval, the rotationalvelocity, and the like. In this embodiment, effects can be obtained bymixing for about 10 minutes or more.

Radicals are produced by cutting the molecules of the elastomer 30 byperforming the second mixing step, and the aggregated carbon nanofibers40 are separated so that the carbon nanofibers are removed one by one bythe elastomer molecules, whereby a carbon fiber composite material inwhich the carbon nanofibers 40 are uniformly dispersed in the elastomer30 at a nano level, as shown in FIG. 3, is obtained. Since the dispersedcarbon nanofibers 40 are prevented from reaggregating due to bondingwith the radicals of the elastomer molecule, excellent dispersionstability is obtained.

Third Mixing Step

The carbon fiber composite material obtained by the second mixing stepis supplied to an open roll set at the first temperature, and a thirdmixing step (tight milling step) is performed a plurality of times, suchas 10 times, to perform sheet forming. The third mixing step may beperformed as required. The roll interval (nip) is set at 0.5 mm or less,such as 0.3 mm, at which the shear force becomes higher than the shearforce in the first and second mixing steps. The roll temperature is setat a third temperature of 0 to 50° C., and still more preferably 5 to30° C. in the same manner as in the first mixing step. The third mixingstep is a final dispersion step for further uniformly dispersing thecarbon nanofibers 40 in the elastomer 30, and is effective when moreuniform dispersibility is required. The elastomer 30 in which theradicals are produced functions to remove the carbon nanofibers 40 oneby one by performing the third mixing step (tight milling step), wherebythe carbon nanofibers 40 can be further dispersed. In addition, acrosslinking agent may be added in the third mixing step to uniformlydisperse the crosslinking agent.

As described above, the carbon nanofibers can be dispersed over theentire elastomer by applying a high shear force by performing the firstmixing step at the first temperature, and the carbon nanofiberaggregates can be disentangled by the radicals of the elastomermolecules by performing the second mixing step at the secondtemperature. Therefore, the carbon nanofibers can be dispersed over theentire nonpolar elastomer such as EPDM, and a carbon fiber compositematerial in which the carbon nanofiber aggregates do not exist can beproduced.

In the first and second mixing steps for dispersing the carbonnanofibers in the elastomer by applying a shear force, it is preferableto use an internal mixer from the viewpoint of processability. As theinternal mixer, a tangential or intermeshing mixer such as a Banbburymixer, a kneader, or a Brabender may be employed. The first, second, andthird mixing steps may be performed using a multi-screw extrusion mixingmethod (twin-screw extruder) instead of the above-described internalmixing method and the open-roll method. The mixer may be appropriatelyselected in combination depending on the amount of production and thelike.

After the first and second mixing steps for dispersing the carbonnanofibers in the elastomer to mix the carbon nanofibers and theelastomer and the third mixing step which is optionally performed, anextrusion step, a molding step, a crosslinking step, and the like may beperformed using a known method.

In the first, second, and third mixing steps for mixing the elastomerand the carbon nanofibers or in the preceding or subsequent step, acompounding ingredient usually used in the processing of an elastomersuch as rubber may be added. As the compounding ingredient, a knowncompounding ingredient may be used. As examples of the compoundingingredient, a crosslinking agent, vulcanizing agent, vulcanizationaccelerator, vulcanization retarder, softener, plasticizer, curingagent, reinforcing agent, filler, aging preventive, colorant, and thelike can be given. In particular, the elastomer can be easily reinforcedby inexpensive carbon black by adding and mixing the carbon black.Moreover, the carbon nanofibers can be more uniformly dispersed bycomplicated flows around the carbon black during the first, second, andthird mixing steps.

(D) Carbon Fiber Composite Material Obtained by Above-Described Method

In the carbon fiber composite material in this embodiment, the carbonnanofibers are uniformly dispersed in the elastomer as the matrix. Thedispersion state of the carbon nanofibers may be evaluated by the valuesof tensile strength (TB), elongation at break (EB), and 100% tensilestress (M100) of the crosslinked carbon fiber composite material.

The tensile strength (TB) and the tensile stress (rigidity) (M100) aregenerally improved by using the carbon nanofibers as the reinforcingmaterial. Moreover, the tensile strength (TB) and the tensile stress(M100) can be further improved by improving the dispersion state of thecarbon nanofibers in the carbon fiber composite material as described inthis embodiment. In particular, since the carbon nanofibers aredispersed over the entire elastomer by mixing the elastomer and thecarbon nanofibers at a comparatively low first temperature, the tensilestrength (TB) and the elongation at break (EB) are improved. However,since the carbon nanofiber aggregates exist, the periphery of theaggregate functions as a fracture starting point, whereby the tensilestress (M100) is not improved to a large extent. However, the carbonnanofiber aggregates are disentangled by mixing the elastomer and thecarbon nanofibers at the second temperature (e.g. 50 to 150° C.) whichis higher than the first temperature, whereby the tensile stress (M100)is improved.

Therefore, the carbon fiber composite material in this embodiment is amaterial of which the values of the tensile strength (TB), elongation atbreak (EB), and tensile stress (M100) are improved and well-balanced. Inthe carbon fiber composite material in this embodiment, the carbonnanofibers are uniformly dispersed without aggregating, since the valuesof the tensile strength (TB), elongation at break (EB), and tensilestress (M100) are improved and well-balanced.

The carbon fiber composite material in this embodiment may be used as anelastomer material or a raw material for a metal or resin compositematerial or the like, as described above. The carbon nanofibers aregenerally entangled and dispersed in a medium to only a small extent.However, when using the carbon fiber composite material in thisembodiment as a raw material for a metal or resin composite material,since the carbon nanofibers exist in the elastomer in a dispersed state,the carbon nanofibers can be easily dispersed in a medium by mixing theraw material with a medium such as a metal or a resin.

Examples of the invention and comparative examples are described below.However, the invention is not limited to the following examples.

EXAMPLES 1 TO 3 AND COMPARATIVE EXAMPLES 1 TO 4 (1) Preparation ofSamples of Examples 1 to 3

An elastomer shown in Table 1 and a predetermined amount of carbonnanofibers were mixed using an internal mixing method to preparesamples.

(a) A Brabender (chamber temperature: 20° C.) as an internal mixer wascharged with a predetermined amount (100 g) of an elastomer (100 partsby weight (phr)) shown in Table 1. As the elastomer, EPDM (EP22)manufactured by JSR Corporation was used.

(b) Carbon nanofibers were added to the elastomer in an amount shown inTable 1 (10 parts by weight (phr)). 1 part by weight (phr) of stearicacid was also added to the elastomer. In Example 3 and ComparativeExample 4, 60 parts by weight (phr) of carbon black was added beforeadding the carbon nanofibers. As the carbon nanofibers, carbonnanofibers with an average diameter of 13 nm manufactured by ILJIN wasused. As the carbon black, SRF carbon black with an average particlediameter of 66 nm was used.

(c) After the addition of the carbon nanofibers, the mixture of theelastomer and the carbon nanofibers was subjected to a mixing(breakdown) step, and removed from the rotors.

(d) The mixture obtained by (c) was placed between the rotors of theinternal mixer set at a temperature of 20° C., subjected to a firstmixing step for a mixing time shown in Table 1, and removed from therotors.

(e) The mixture obtained by (d) was placed in an internal mixer set at asecond temperature shown in Table 1, subjected to a second mixing stepfor a mixing time shown in Table 1, and removed from the internal mixer.

(f) The mixture obtained by (e) was supplied to six-inch an open rollwith a narrow roll interval (nip) of 0.3 mm and a roll temperature of20° C., and tight-milled ten times (third mixing step). The tightmilling was repeatedly performed ten times. In the tight milling step, 2parts by weight (phr) of a peroxide was added as a crosslinking agent.The tight-milled mixture was rolled to a thickness of 1.1 mm andremoved.

(g) The sample cut into a die size was placed in a die and subjected topress-crosslinking at 175° C. and 100 kgf/cm² for 20 minutes to obtain acrosslinked sheet with a thickness of about 1.0 mm.

The samples of Examples 1 to 3 were thus obtained.

As Comparative Example 1, a sample was formed using only EPDM. InComparative Example 2, a sample was obtained without performing thefirst and second mixing steps (steps (d) and (e)). In ComparativeExamples 3 and 4, the samples were obtained without performing thesecond mixing step (step (e)).

(2) Measurement of Tensile Strength (TB), Elongation at Break (EB), andTensile Stress (M100)

The TB, EB, and M100 of the samples of Examples 1 and 2 and ComparativeExamples 1 to 3 were measured in accordance with JIS K 6521-1993. Themeasurement results are shown in Table 1.

TABLE 1 Comparative Comparative Comparative Comparative Example 1Example 2 Example 3 Example 1 Example 2 Example 3 Example 4 Type ofelastomer EPDM EPDM EPDM EPDM EPDM EPDM EPDM Mixing ratio Elastomer(phr) 100 100 100 100 100 100 100 Carbon nanofibers (phr) 10 10 10 0 1010 10 SRF carbon black (phr) 0 0 60 0 0 0 60 Firtst First temperature (°C.) 20 20 20 — — 20 20 kneading step Time (min) 10 10 10 — — 10 10Second Second temperature (° C.) 60 100 100 — — — — kneading step Time(min) 10 10 10 — — — — Third Third temperature (° C.) 20 20 20 — 20 2020 kneading step Tight milling (times) 10 10 10 — 10 10 10 TB (Mpa) 9.59.6 27.2 1.5 5.1 6.9 19.4 EB (%) 125 145 135 175 135 170 120 M100 (Mpa)6.6 7.4 24.3 1.1 3.9 3.5 15.1

From the results shown in Table 1, the following items were confirmedaccording to Examples 1 to 2 of the invention. Specifically, the carbonfiber composite material containing the carbon nanofibers has higher TBand M100 values in comparison with the EPDM which does not contain thecarbon nanofibers. The TB value of the carbon fiber composite materialof the invention is higher than those of Comparative Examples 2 and 3.The M100 value of the carbon fiber composite material of the inventionis higher than those of Comparative Examples 2 and 3. These resultssuggest that the carbon nanofibers are uniformly dispersed over theentire carbon fiber composite material according to the example and thatthe number of carbon nanofiber aggregates is small. According to theexample of the invention, it was confirmed that the tensile strength(TB) and the tensile stress (M100) are improved while maintaining theelongation at break (EB) due to inclusion of the carbon nanofibersuniformly dispersed in the matrix, so that the reinforcing effect due tothe carbon nanofibers is obtained. The TB, EB, and M100 are alsoimproved when mixing the carbon black as in Comparative Example 4.However, the TB and M100 are improved in a well-balanced manner inExample 3 while maintaining the EB.

A scanning electron microscope (SEM) image was taken for the sample ofthe carbon fiber composite material obtained in Example 2. FIG. 4 showsthe resulting SEM image. The photographing conditions were set at anacceleration voltage of 3.0 kV and a magnification of 10.0 k. From theSEM image shown in FIG. 4, it was confirmed that the carbon nanofibersare separated and uniformly dispersed in the EPDM. In FIG. 4, whitelinear sections indicate the carbon nanofibers.

As a reference, FIG. 5 shows an SEM image of the sample of ComparativeExample 3. The SEM photographing conditions were set at an accelerationvoltage of 3.0 kV and a magnification of 10.0 k. As shown in the SEMimage in FIG. 5, although the carbon nanofibers are dispersed over theentire material, a number of small entangled aggregates are scatteredover the material.

As described above, according to the invention, it was confirmed thatthe carbon nanofibers, which can be generally dispersed in a matrix toonly a small extent, are uniformly dispersed in the elastomer,particularly in a nonpolar elastomer such as EPDM.

Although only some embodiments of the present invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the embodimentswithout materially departing from the novel teachings and advantages ofthis invention. Accordingly, all such modifications are intended to beincluded within scope of this invention.

1. A carbon fiber composite material obtained by a method comprising: afirst mixing step of mixing an elastomer and carbon nanofibers at afirst temperature; a second mixing step of mixing a mixture obtained bythe first mixing step at a second temperature; and a third mixing stepof mixing the carbon fiber composite material obtained by the secondmixing step at a third temperature; wherein: the first temperature is 50to 100° C. lower than the second temperature; the elastomer is ethylenepropylene rubber (EPDM); the third temperature is lower than the secondtemperature; and the third mixing step comprises performing tightmilling a plurality of times by using an open roll with a rotor intervalof 0.5 mm or less.
 2. The carbon fiber composite material as defined inclaim 1, wherein the elastomer has a molecular weight of 5,000 to5,000,000.
 3. The carbon fiber composite material as defined in claim 1,wherein: the first temperature is 0 to 50° C.; and the secondtemperature is 50 to 150° C.
 4. The carbon fiber composite material asdefined in claim 1, wherein the carbon nanofibers have an averagediameter of 0.5 to 500 nm.
 5. The carbon fiber composite material asdefined in claim 1, wherein the first mixing step comprises mixing theelastomer and the carbon nanofibers together with carbon black.